1. Field of the Invention
The invention relates generally to a current-perpendicular-to-the-plane (CPP) magnetoresistive sensor that operates with the sense current directed perpendicularly to the planes of the layers making up the sensor stack, and more particularly to an improved method for forming the reference layer of the CPP sensor.
2. Background of the Invention
One type of conventional magnetoresistive (MR) sensor used as the read head in magnetic recording disk drives is a “spin-valve” sensor based on the giant magnetoresistance (GMR) effect. A GMR spin-valve sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically formed of Cu or Ag. One ferromagnetic layer, typically called the “reference” layer, has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic “pinning” layer, and the other ferromagnetic layer, typically called the “free” layer, has its magnetization direction free to rotate in the presence of an external magnetic field. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the fixed-layer magnetization is detectable as a change in electrical resistance. If the sense current is directed perpendicularly through the planes of the layers in the sensor stack, the sensor is referred to as current-perpendicular-to-the-plane (CPP) sensor.
In a magnetic recording disk drive CPP-GMR read sensor or head, the magnetization of the fixed or pinned reference layer is generally perpendicular to the plane of the disk, and the magnetization of the free layer is generally parallel to the plane of the disk in the absence of an external magnetic field. When exposed to an external magnetic field from the recorded data on the disk, the free-layer magnetization will rotate, causing a change in electrical resistance.
The reference layer in a CPP-GMR sensor used in read heads may be a single pinned layer (sometimes called a “simple” pinned layer), in contrast to the well-known antiparallel (AP) pinned structure. In a simple pinned structure the reference layer has its magnetization pinned by being exchange-coupled to an antiferromagnetic (AF) pinning layer, which is typically a Mn alloy like IrMn. However, CPP-GMR sensors are hampered by reference layer instability. The signal to noise ratio (SNR) of CPP-GMR sensors is limited by spin-torque generated fluctuations of the magnetizations of the free and reference layers. The free layer can be made more stable to such spin-torque instability by forming an antiferromagnetically coupled (AFC) ferromagnet/Ru/ferromagnet trilayer, also called an antiparallel (AP) free layer. In contrast, forming such a structure for the reference layer actually increases fluctuations due to spin-torque since the AP-reference layer generally possesses lower magnetic damping compared to a simple-pinned reference layer. Thus the ideal combination for highest SNR in CPP-GMR sensors, all other factors being equal, is an AP-free layer and a simple-pinned reference layer. The simple-pinned reference layer structure is a film stack of seed layers/IrMn AF layer/ferromagnetic reference layer. An ideal reference layer is not only well-coupled to the AF layer, but also has a small coercivity so that only one orientation of the reference layer magnetization exists at low magnetic fields where the sensor will operate. However, the prior art processes for forming the simple-pinned reference layer structure are not only time-consuming but also do not produce a reference layer with the desired low coercivity.
What is needed is an improved method for making a CPP-GMR sensor with a simple-pinned reference layer that is well-coupled to the AF layer and that has low coercivity.